The present invention relates to a heating-type toner-fixing unit to be used in an image forming machine such as a fax machine, a copying machine, a printer and the like.
Formerly, in heating-type toner-fixing units used in image forming machines such as fax machines, copying machines, printers and the like, after a toner image formed on a photosensitive drum is transferred onto a sheet of paper or the like as a recording material, the toner image is fixed on the sheet surface by heating and pressing by the toner-fixing unit. These toner-fixing units comprise a heating roller and a resin pressure roller, and of these, the heating roller employs a system where a cylindrical metal roller is provided therein with a heat source such as a halogen lamp or the like and the surface region of the metal roller is heated by the heat of the heat source so as to fix the toner.
In recent years, a heating-type toner-fixing unit using a ceramic heater has been proposed for this fixing system, and put into practical use. This method has been disclosed in Japanese Unexamined Patent Publication No. Hei-1-263679, Japanese Unexamined Patent Publication No. Hei-2-157878, and Japanese Unexamined Patent Publication No. Sho-63-313182, for example. Specifically, a ceramic heater is attached to a resin heater support, furthermore, a heat-resistant fixing film revolves around the periphery thereof at a speed almost the same as that of a pressure roller to feed a sheet of paper, and heat of the ceramic heater is transferred via the fixing film to the toner and the sheet of paper. In this method, there are advantages such that, since heat capacity of the ceramic heater as a heating element is significantly smaller than that of the former metal roller, power consumption can be reduced and moreover, since preheating of the heater after power is supplied thereto is unnecessary, the quick-start property is excellent. Currently, as a base material of the ceramic heater, alumina (Al2O3) is generally used.
A further detail is described here based on FIG. 1 and FIG. 2, which conceptually show this fixing method. A ceramic heater 1 is attached to a heater support 2, around the periphery of which a cylindrical heat-resistant fixing film 3 is movably disposed, and a pressure roller 4 synchronously rotates while pressing the fixing film 3 against the ceramic heater 1. The fixing film 3 which is pressed against the ceramic heater 1 by the pressure roller 4 forms a nip portion having a nip width of N between the ceramic heater 1 and the pressure roller 4 as a result of the peripheral deformation of the pressure roller 4 as shown in FIG. 2 in an enlarged manner. A recording material 5 such as a sheet of paper on which a toner image 6a is formed is inserted between the rotating fixing film 3 and pressure roller 4, and the toner image 6a is heated and pressed at the nip portion and fixed as an image 6b on the recording material 5.
In addition, the ceramic heater 1 has, for example, a structure as shown in FIG. 3. Namely, one or a plurality of heating elements 1b and current-conducting electrode(s) 1c for conducting current to these heating elements 1b are provided on a ceramic base plate 1a, and an overcoat glass layer 1d for protection and for securing insulation is formed on the heating elements 1b. This ceramic base plate 1a generally has a thin rectangular flat-plate shape as a whole, and the heating elements 1b are formed on a fixing face, or the back of the fixing face, where the ceramic base plate 1a comes into contact with the pressure roller 4 via the fixing film 3.
Recently, a higher fixing rate has also been demanded for this fixing method. In a conventional ceramic heater using an alumina base plate, the fixing rate is 4 to 16 ppm (4 ppm is a rate for feeding four pages of A4-size paper per minute), and furthermore, high-speed processing at 24 ppm or more has been demanded.
However, the above case of a ceramic heater using an alumina base plate is disadvantageous in terms of base plate fractures caused by thermal shock during rapid heating. Namely, a voltage of 100V or 200V is applied to one or both ends of the heating elements of the ceramic heater to generate Joule heat of several hundreds of watts or more, thereby rapidly raising the temperature to approximately 200xc2x0 C. in approximately 2 to 6 seconds, and therefore, the alumina base fractures. In addition, when the fixing rate is increased, the time needed to transfer heat from the heater to each sheet of paper is shortened. Since a certain amount of heat is necessary to fix the toner, however, a greater amount of heat must be supplied from the heater to the sheet per unit time. As a result, the thermal shock to be applied to the heater tends to be increased, thus increasing the probability of ceramic base plate fracture.
Therefore, ceramic heaters using aluminum nitride (AlN) excellent in thermal shock resistance have been disclosed in Japanese Unexamined Patent Publication No. Hei-9-80940 and Japanese Unexamined Patent Publication No. Hei-9-197861. According to the descriptions of Japanese Unexamined Patent Publication No. Hei-9-80940, by exploiting the fact that aluminum nitride has a higher thermal conductivity than that of aluimina, temperature responsivity of the heater is improved. According to the heater described in Japanese Unexamined Patent Publication No. Hei-9-197861, by utilizing the high thermal conductivity of aluminum nitride, an improvement in the fixing quality and high-speed printing is realized and power consumption is reduced.
For realization of a higher fixing rate, a problem has been pointed out in terms of not only the material (thermal conductivity and thermal expansion coefficient) of the ceramic heater as a heat source as in the above but also the shape thereof. Namely, as the fixing rate increases, the inner circumferential surface of the fixing film, which revolves at a high speed, is abraded due to friction between the ceramic heater and heater support and due to abrasion powder, lubricity of a lubricant such as grease which is applied to the fixing film and the ceramic heater thereof is lost, thereby disabling revolution of the fixing film.
In respect of this problem, an improvement has been attained as a result of investigations of the inventors, et al. of the present invention, by forming a curved surface on the ceramic heater, for which patent applications were already filed as Japanese Patent Application No. 2000-136621, Japanese Patent Application No. 2000-239280, and Japanese Patent Application No. 2000-239281. A higher rate and further improvement in durability of the fixing film have been demanded
In light of such prior circumstances, it is an object of the present invention to provide a heating-type toner-fixing unit which can still further improve durability of a fixing film and also further improve fixing quality during high-rate fixing at 24 ppm or more.
In order to achieve the above object, a first aspect of the present invention provides a heating-type toner-fixing unit in which a cylindrical heat-resistant fixing film is pressed against a heating member by means of a pressure member to form a nip portion between the fixing film and heating member whereby fixing is carried out by heating a recording material carrying an unfixed toner image while nipping and feeding this recording material, wherein the shape of the fixing face-side surface of a ceramic heater as the heating member which comes into contact with the fixing film and the shape of portions of a heater support at least adjacent to the fixing face-side surface are formed into a shape that is almost identical to a naturally deformed shape of the fixing film in a static state where the fixing film is pressed at a designated nip width against the heating member by the pressure member.
In addition, a second aspect of the present invention provides a heating-type toner-fixing unit in which a cylindrical heat-resistant fixing film is pressed against a heating member by means of a pressure member to form a nip portion between the fixing film and heating member and fixing is carried out by heating a recording material carrying an unfixed toner image while nipping and feeding this recording material, wherein the shape of the fixing face-side surface of a ceramic beater as the heating member which comes into contact with the fixing film and the shape of portions of a heater support at least adjacent to the fixing face-side surface are formed into a shape that is almost identical to a naturally deformed shape of the fixing film in a traveling state where the fixing film is pressed against the heating member by the pressure member at a designated nip width so as to travel.
In the heating-type toner-fixing unit according to the above first and second aspects of the present invention, the shape of the fixing face-side surface of the ceramic heater which comes into contact with a fixing film of the ceramic heater and the shape of portions of the heater support at least adjacent to the fixing face-side surface are formed into a curved surface which is generally convex or concave with respect to the pressure member. In addition, the shape of the fixing face-side surface of the ceramic heater which comes into contact with a fixing film and the shape of portions of the heater support at least adjacent to the fixing face-side surface can be, from the entrance of the nip portion to the center of the nip portion, a shape which is almost identical to a naturally deformed shape of the fixing film in a static state or a traveling state where the fixing film is pressed against the heating member by the pressure member and, from the center of the nip portion to the exit of the nip portion, a flat shape.
Furthermore, a third aspect of the present invention provides a heating-type toner-fixing unit in which a cylindrical heat-resistant fixing film is pressed against a heating member by means of a pressure member to form a nip portion between the fixing film and heating member and fixing is carried out by heating a recording material carrying an unfixed toner image while nipping and feeding this recording material, wherein the shape of the fixing face-side surface of a ceramic heater as the heating member which comes into contact with the fixing film and the shape of portions of a heater support at least adjacent to the fixing face-side surface are, at the nip portion and at the entrance side and exit side adjacent to the nip portion, formed into a flat shape parallel to the nip portion, and at other portions, are formed into a shape along the cylindrical shape of the fixing film.
In the heating-type toner-fixing unit according to the above first through third aspects of the present invention, the main constituent of the ceramic heater is aluminum nitride or silicon nitride.